1. Field of the Invention
The present invention relates to a lead frame used in ICs and the like which is formed out of nickel or a nickel alloy, copper or a copper alloy, and iron or an iron alloy. In more particular, it relates to semiconductor devices having tin-based solder film which does not contain lead, one of the environmentally harmful pollutants, and a process for producing the devices.
2. Description of the Related Art
The structure of the lead frame of the prior art is shown in FIGS. 3 and 4. FIGS. 3 and 4 respectively show the plan view and the sectional view of the lead frame of the prior art. In the lead frame of the prior art, the inner lead part 2 to which wire bonding is to be applied is coated with silver plating 5 and the outer lead part 1 is provided with a surface treatment layer containing lead and tin.
In recent years, environmental problems have been attracting much attention. Under the situations, also with regard to the parts used in IC packages, investigations are being made to obtain a usable material which does not contain environmentally harmful substances. Among the materials used in lead frames for electronic parts, one of the materials regarded as particularly harmful to environment is lead used in solder. When allowed to stand, lead will dissolve out into environment and exert a bad influence on a human body, so that, in the electronic industry, efforts are being made to develop lead-free solder, soldering paste, etc. However, no material has yet reached the stage of practical usage which has characteristic properties equal or superior to those of the present-day lead-containing solder. Various attempts have been made with regard to lead frames used for electronic parts. In recent years, lead frames fully plated with palladium to replace lead-containing solder have come to be used in practice. However, when palladium is used singly, on application of heat in the die attaching step or wire bonding step, the wettability of solder tends to deteriorate and the reliability of soldering in surface packaging tends to be insufficient. In recent years, accordingly, products which have thin gold plating as a protective film on the palladium surface have been proposed.
On the other hand, attempts other than the use of palladium are being made at present to form lead-free solder plating by incorporation of such metals as indium, bismuth and zinc in place of lead used in the present-day tin-lead based solder. In the solder alloys and soldering paste used for reflowing, ternary alloys or quaternary alloys which contain, in addition to tin, two or more kinds of metals have been proposed. However, since it is difficult to control the deposition composition of ternary or quaternary alloys in a plating liquor, binary alloys which contain tin and one other metal are currently predominant.
Recently, alloys of tin incorporated with silver have attracted attention as the most promising candidate for lead-free solder, and the plating liquor for such an alloy is being developed. However, the alloy has not yet been used in practice owing to such problems as unsatisfactory appearance, development of cracks caused by bending, discoloration caused by thermal hysteresis and deterioration of solder wettability caused by thermal hysteresis.